Electrochemical fuel cells convert fuel and oxidant into electricity and reaction product. With reference to FIG. 1, an electrochemical fuel cell 10 generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) 12 comprising a solid polymer electrolyte or ion exchange membrane (xe2x80x9cIEMxe2x80x9d)14 disposed between two electrode layers or substrates 16A, 16B formed of electrically conductive sheet material. The electrode substrate has a porous structure that renders it permeable to reactants and products in the fuel cell. The MEA also includes an electrocatalyst, typically disposed in a layer at each ion exchange membrane/electrode substrate interface, to induce the desired electrochemical reaction in the fuel cell. The electrodes are electrically coupled to electrical charge collecting plates to provide a path for conducting electrons between the electrodes through an external load. At the anode, the fuel stream moves through the porous anode substrate and is oxidized at the anode electrocatalyst. At the cathode, the oxidant stream moves through the porous cathode substrate and is reduced at the cathode electrocatalyst.
In electrochemical fuel cells employing hydrogen, hydrogenated inert gas or other hydrogen rich agent as the fuel and oxygen or air as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane, which is also known as the proton exchange membrane (xe2x80x9cPEMxe2x80x9d), facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product.
In electrochemical fuel cells, the MEA is typically interposed between two separator plates or reactants flow field plates (anode and cathode plates) 18A, 18B. The plates typically act as current (electrical charge) collectors and provide support to the MEA. Reactants flow field plates typically have channels, grooves or passageways 20 formed therein to provide means for access of the fuel and oxidant streams to the porous anode and cathode substrates, respectively.
The electrode substrates of an MEA, which are also known as the porous diffusion substrate or diffusion substrate, are responsible for: (1) delivering active fuel agents such as hydrogen or methanol from the channels defined by the the flow field plates to the surface of the PEM membrane; (2) providing a flow path for electrical charge from the PEM or ion exchange membrane surface to an electrically conductive charge collecting plate, typically interfaced to or part of the metallic body of the active fuel agent flow field plate; (3) chemically insulating of the charge collecting plate or the metallic flow field plate from the reactive surface of the PEM or IEM membrane; and providing for proper moisturizing of PEM or IEM membrane and removal of reaction products.
The material used for the porous electrode substrate or diffusion substrate is electrically conductive to provide a conductive path between the electrocatalyst reactive sites and the charge or current collectors. There are a great numbers of substances qualifying as a sufficiently good for manufacturing the porous electrode substrate. All of them however exhibit one or more negative characteristics, such as: (1) substantial resistance to electric current flow; (2) degradation and corrosion as a result of exposure to reactants, catalysts and ion exchange membrane in fuel cell operation; (3) poor mechanical characteristics like integrity, uniformity or stiffness; (4) complex construction involving the combination of several components; and (5) difficulty in optimizing for high permeability for the reactants and low electrical resistance, which are mutually opposed parameters.
At present, a preferred type of porous diffusion substrate for reactive agents is formatted with polymer or carbon fiber cloth or paper saturated with graphite particles. Such substrate is characterized by: (1) low porosity or highly variable porosity that restricts the flow of active fuel agents and requires relatively high positive pressure to enable the flow of the active fuel agents; (2) a high electrical cross-resistance that causes substantial power loss and heat dissipation as a result of electric current flow through; (3) low mechanical stability of the graphite particles that in the long run can contribute to unsafe operational conditions and internal electrical shorts; and (4) mechanical compressibility that causes substantial performance variation through variation in the electrical cross-conductance, variation in the resistance to the flow of active fuel agents, variation in the quality of contact with PEM, and potential blockage of flow field channels.
Some of those negative characteristics are partially negotiated by an intermediate metallic grid inserted between the diffusion substrate and ridges of charge collecting plate of the flow field metal plates. The positive electrical benefits of the grid are not completely realized due to the relatively high electrical resistance of the diffusion substrate.
The present invention provides a solution to the problem of the internal power loss in fuel cell assembly attributable to the electrical resistance of present diffusion substrates.
The diffusion substrate is comprised of a base and bed structure that has relatively low-cross plane resistance, relatively stable porosity for reactant flow, mechanical characteristics that promote low cross-plane resistance and stable porosity, and resistance to degrading influences.
In one embodiment, the bed of the diffusion substrate is comprised of a plurality of micro-studs that are operatively attached to the base, capable of conducting electrical charge, and extend between two surfaces. Additionally, the bed includes a plurality of channels that extend between the two surfaces and provide paths for the reaction constituents. The micro-studs can take several forms. In one embodiment, the micro-studs are open-ended, nanotubes (xe2x80x9cNTsxe2x80x9d), such as single walled nanotubes (xe2x80x9cSWNTsxe2x80x9d) and multi-wall nanotubes (xe2x80x9cMWNTsxe2x80x9d). With open-ended nanotubes, at least some of the channels that are used to transport the reaction constituents are realized by the holes extending between the open ends of the nanotubes. Additionally, the open-ended nanotubes are capable of conducting the electrical charge resulting from the operation of a fuel cell. In one embodiment, open-ended, carbon nanotubes are used because of their low axial resistance. However, open-ended nanotubes made of other conductive material are also feasible.
Other embodiments of the diffusion substrate utilize nanotube ropes and fiber studs. In these embodiments, at least some of the channels that are used to transport the reaction constituents are the interstices situated between the ropes or fiber studs. The nanotube ropes and fiber studs are capable of conducting the electrical charge produced by the fuel cell and, at least in one embodiment, are made of carbon.
In a further embodiment, the diffusion substrate utilizes a bed that is comprised of a plurality of columnar elements that are operatively attached to the base and extend between two surfaces. The columnar elements also have longitudinal axes that are substantially parallel to one another and are capable of conducting electrical charge. In one embodiment, the columnar elements are substantially perpendicular to the at least one of the noted surfaces. In various embodiments, the columnar elements are implemented with open-ended nanotubes, nanoropes and fiber studs.